Evaluation of a mesoscale atmospheric dispersion modeling system with observations from the 1980 Great Plains mesoscale tracer field experiment. Part II: Dispersion simulations

Abstract

The Colorado State University mesoscale atmospheric dispersion (MAD) numerical modeling system, which consists of a prognostic mesoscale meteorological model coupled to a mesoscale Lagrangian particle dispersion model (MLPDM), has been used to simulate the emission, transport, and diffusion of a perfluorocarbon tracer-gas cloud for one afternoon surface release during the July 1980 Great Plains mesoscale tracer field experiment. The MLPDM was run for a baseline simulation and seven sensitivity experiments. The baseline simulation showed considerable skill in predicting such quantitative whole-cloud characteristics as peak ground-level concentration (GLC), maximum cloud width, cloud arrival and transit times, and crosswind integrated exposure at downwind distances of both 100 and 600 km. The baseline simulation also compared very favorably to simulations made by seven other MAD models for this same case in an earlier study. The sensitivity experiments explored the impact of various factors on MAD, especially the diurnal heating cycle and physiographic and atmospheric inhomogeneities, by including or excluding them in different combinations. The GLC "footprints" predicted in the sensitivity experiments were sensitive to differences in the simulated meteorological fields. The observations and the numerical simulations both suggest that the Great Plains nocturnal low-level jet played an important role in transporting and deforming the perfluorocarbon tracer cloud during this MAD experiment: the mean transport speed was supergeostrophic and both crosswind and alongwind cloud spreads were larger than can be explained by turbulent diffusion alone. The contributions of differential horizontal advection and mesoscale deformation to MAD dominate those of small-scale turbulent diffusion for this case, and Pasquill's delayed-shear enhancement mechanism for horizontal diffusion appears to have played a significant role during nighttime transport. These results demonstrate the need in some flow regimes for better temporal resolution of boundary layer vertical shear in MAD models than is available from the conventional twice-daily rawinsonde network.